This invention relates generally to irrigation and particularly to drip irrigation tape and its fabrication.
The drip irrigation system is far less noticeable than many of its counterparts. Instead of spraying large amounts of water over the crops, it supplies less water more effectively through lengths of tubing placed near the roots where drops of water emitted through small outlets moisten the soil just the right amount. Crops improve, salt accumulation and fertilizer loss decreases, and water is conserved.
In addition to its low profile and effective performance, this clever and inherently simple technique is easier and less expensive. As a result, it has come into such widespread use that many miles of irrigation tubing are in demand, the generally flat plastic tubing called drip irrigation tape being especially popular.
This type of tubing lies flat for easy handling. It can be rolled up on reels for transportation and to ease distribution over a field. Once water is introduced, it swells like the perforated sprinkling hose used to water the front lawn, and as water flows under pressure within, outlets along the tubing emit drops or water into the soil to water the crops at their roots where most needed and at a rate most beneficial.
Much of today's drip irrigation tape is fabricated from a thin pliable plastic strip folded lengthwise. The two edges are overlapped and joined together to form a flat hollow tape that may be on the order of one inch wide. Under pressure, it swells toward the general shape of cylindrical tubing with an inside diameter that may be about five-eighths inch. This is the main conduit for irrigation water--the main passageway for water flowing to crops under irrigation.
But unlike sprinkling hose, drip irrigation tape also employs a much smaller secondary conduit. It is usually located along the seam formed by the overlapping edges of the plastic strip and it extends along the tape adjacent the main conduit as a narrow passageway that helps reduce the rate at which water is emitted into the soil. Water supplied to the main conduit spends its energy finding its way into the secondary conduit through a series of inlets interconnecting the two, and then forcing its way along the secondary conduit to a series of outlets where it slowly trickles out drop by drop into the soil.
This is the secret of drip irrigation tape that makes it so effective--slow built-in leaks evenly spaced along its length--and the design details accomplishing this are critical to inexpensive manufacture and successful operation in a drip irrigation system. By focusing on these details, we can appreciate some of the problems in existing designs that need to be overcome.
The seam and secondary conduits are especially important. The seam is some designs, for example, is formed by joining the overlapping edges of the plastic strip together with an adhesive applied lengthwise down the tape in two fine parallel lines, the space between the two adhesive beads serving as the secondary conduit. One such design employs thin monofilament fishing line that is first dipped in adhesive and then applied between the edges to form the beads.
While these designs employ the expedient of making the secondary conduit part of the seam, they have certain drawbacks. In the first place, the adhesive beads constitute additional material and additional steps in the fabrication process. They also must be accurately applied and carefully controlled to maintain dimensioning that will achieve a consistent drip rate along the tape.
Furthermore, the secondary conduit is often prone to collapse due to an apparent electrostatic affinity of the overlapping edges for each other. Whether in manufacturing, through handling, or while buried in the middle of a field under irrigation, the secondary conduit often becomes deformed so that the overlapping edges of the plastic strip touch and stick together in the gap between the adhesive beads. When this happens, the secondary conduit becomes blocked and the tape malfunctions.
Other designs have their drawbacks also. For instance, the needle holes of a stitched seam used as outlets in one design and the laser-cut slits in another often become blocked by soil or particulate material in the irrigation water and the tape malfunctions. In addition, fabrication is more involved.
Thus, while existing designs generally employ a two-conduit approach to achieve a slow controlled drip rate, they have certain specific drawbacks related to fabrication, operation, and reliability.
Consequently, it is desirable to have a new and improved drip irrigation tape that alleviates these concerns--one easily and inexpensively manufactured, able to achieve a consistent drip rate along its length, and less susceptible to malfunctioning.